I know there're a lot of HK416 mods here, I wouldn't bother using one only if the SS3 model wasn't corrupted.
The only thing that survive to this is the holographic sight.
Credits
Twinke Masta: HK416 model
ImbrokenRU: animations
Croteam: holographic sight model, sounds
Note: this will conflict with Twinke Masta's AK47 and any mods that change particle effects, but other than that it works fine.